There is a well-known class of lasers, such as CO.sub.2 electric discharge and chemical lasers, for which the gain region generally has a long dimension along the beam and a rectangular or elliptical cross section with a high aspect ratio. Such a region is, of course, not azimuthally isotropic.
It is well known in the field that for optimum transverse mode control in an unstable laser device, it is required that the feedback field (or the electric field in the vicinity of the feedback mirror) possess a half-integral equivalent Fresnel number that is azimuthally isotropic over the feedback mirror.
The problem addressed by the present invention is the reconciliation of these two conflicting requirements.